“Mad Science” Mini-Lessons

7. Newton’s third law

Topic

Forces are always interactions between objects: every force on an object is exerted by another object, and in turn the other object receives a force equal in strength in the opposite direction.

Supplies

Rope, rolling platform such as a skateboard or other low-friction surface; tie-down location

Optional: football

Procedure

Advanced students: Ask students: What is Newton’s third law? (Expected answer: for every action there is an equal and opposite reaction.) Follow up with: What is a force? (Expected answer: a force is a push or pull. Ideal answer: a force is an influence that changes an object’s velocity.)

Beginning students: Ask students: When you kick a football, your foot feels the ball. [Demonstrate kicking a football.] Which force is greater: the force your foot exerts on the ball, or the force the ball exerts back on your foot? (Expected answer: the force exerted on the ball is greater. Correct answer: the two forces are exactly the same strength.) How about when you kick a wall? [Demonstrate kicking a wall.] How would you compare the forces on the wall and on your foot?

Explain Newton’s third law: When any first object exerts a force on a second object, the second object exerts an opposite force on the first object. By “opposite force”, I mean that it has the same strength but is in the opposite direction, and along the same line of interaction.

This has some interesting consequences:

All forces are interactions between objects: there can be no force on an isolated object

Every force exerted on an object must be exerted by another object

So why, when you kick a football, the football speeds away while you stay in place (or keep running)? How does the ball’s mass compare to your mass? How about your mass and the mass of the wall you kick?

Demonstration of the force in the opposite direction

Tie a rope to a light object and pull on the rope, pulling the object to you. Clearly, you apply a force toward you on the object.

Then tie the rope to a fixed object and stand on the skateboard. Pull on the rope as before. This time, it is you that moves—toward the fixed object, opposite the direction that you are pulling! The fixed object is pulling you toward it, just as you pulled the lighter object toward you.

Stubborn ox question:

You hitch your ox to a ngolo and tell it to pull. To your surprise, the ox protests that there is no point in pulling, because the ngolo will pull backward on him just as hard as he pulls forward on the ngolo. What is wrong with his thinking?

Wrap-up question:

Which force is stronger: the pull of the earth’s gravity on the Moon, or the pull of the Moon’s gravity on the earth? (Correct answer: they are equal in strength although in opposite directions, each toward the other body.)